Instrumentation & Measurement Magazine 24-3 - 21

Fig. 4. The diagram of the NRC precision ac source.

can be used, allowing a variable full scale with 20-bit amplitude resolution to be attained. Fig. 4 shows the schematics of
the source.
Digital parts are represented by blocks in dark blue comprising a clock conditioner and internal 10 MHz time-base
delivering a reference clock for a DDS of 48-bit, for generating a 12 MHz master clock for an ARM-Cortex M7 processor.
The M7 processor generates the code for ac signal synthesis in
the DAC, a zero-crossing signal 'ZC OUT', the sampling frequency 'Fs OUT' and a reduced divided by a programmable
integer m output signal 'Fs/m OUT' for sampling purposes as
shown. Digital and analog parts are isolated by optical isolators (ISO). The microcontroller serves as the interface for a PC
via an USB port and controls other parts of the system. The

dc-path is shown in light
blue blocks. The 20-bit resolution DAC generates a
variable dc voltage for controlling the amplitude of
the ac signal at the output.
The ac path is represented
in dark orange and is
controlled by the M7 processor. Second harmonic
distortion is adjustable to
negligible values by controlling the offset of the
chopped dc reference for
the DAC, which is buffered
by an audio amplifier like
the dc reference voltage as
shown.
Due to its outstanding
characteristics, it will extensively be used in calibrations with Josephson standards and
digital impedance bridges, since it allows phase adjustments
up to the nano-radian range to be done, yielding perfectly
phase alignment of signals (a need with Josephson systems)
and phase balancing of a new digital impedance bridge under
development at the NRC.
Fig. 5 shows the plot of Allan standard deviations for
amplitude measurements at the ac source output via digital sampling with a commercial digital voltmeter (Keysight
3458A) for a 1 kHz signal of 10 V amplitude. Note that with
a moderate number of averages τ of nearly ten, the attainable standard deviations of measurement are of the order of
0.1 μV/V (or 1 part in 10+7 V/V).
For frequencies up to 100 Hz, the attainable Allan standard
deviations are typically three times smaller. The insert in Fig.
5 illustrates the front of one of the NRC ac sources used for the
measurements.
Future work on digital signal synthesis will be concentrated on frequencies above 20 kHz for the synthesis of stable
ultra-pure waveforms aided by digital signal processors.

Conclusions and Outlook

Fig. 5. Allan standard deviations of the output signal of the NRC source
#B (shown in the insert) at 1 kHz and 10 V peak from measurements using a
commercial sampling digital voltmeter (Keysight 3458A). Each point in the
figure corresponds to a measurement time of five seconds, i.e., the number of
averages denoted by τ corresponds to a multiple of five seconds. Note that the
graphic contains noise contributions of the voltmeter, which were not subtracted
from measurements. Noise is essentially white up to a τ of around 15.
May 2021

The paper presented ongoing R&D work in the field of AC
Metrology and provided an overview on some challenges
being addressed. We expect in the near future to attain uncertainties of measurement of some parts in 10+8 V/V up to
1 kHz and extend the application of the NRC Josephson systems far into the mega-Hertz frequency range. New hardware
was added to the system, and future developments will allow extracting even better accuracies of the NRC Josephson
standards. Future tasks shall concentrate on mathematical
modeling and hardware development with fast programmable logic devices and digital signal processors. Besides using
adaptive filtering, we also aim at adding software components of artificial intelligence in our digital system for control
and data analysis.

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